Places bring new support for message-passing parallelism to Racket. This paper gives an overview of the programming model and how we had to modify our existing, sequential runtime-system to support places. We show that the freedom to design the programming model helped us to make the implementation tractable; specifically, we avoided the conventional pain of adding just the right amount of locking to a big, legacy runtime system. The paper presents an evaluation of the design that includes both a real-world application and standard parallel benchmarks.

Many language implementations, particularly for high-level and scripting languages, are based on carefully honed runtime systems that have an internally sequential execution model. Adding support for parallelism in the usual form -- as threads that run arbitrary code in parallel -- would require a major revision or even a rewrite to add safe and efficient locking and communication. We describe an alternative approach to incremental parallelization of runtime systems. This approach can be applied inexpensively to many sequential runtime systems, and we demonstrate its effectiveness in the Racket runtime system and Parrot virtual machine. Our evaluation assesses both the performance benefits and the developer effort needed to implement our approach. We find that incremental parallelization can provide useful, scalable parallelism on commodity multicore processors at a fraction of the effort required to implement conventional parallel threads.